Optical switching apparatus and optical communication network system

ABSTRACT

An optical communication network system is disclosed, which includes an optical switching apparatus. The optical switching apparatus includes an optical switch for switching and setting routes of an optical signal without being converted, a control unit for instructing the optical switch to execute a route switching operation, and a performance monitor for detecting performance of the optical signal having a route set by the optical switch. The performance monitor issues an alarm when the performance detected is deteriorated from predetermined performance. The control unit includes an alarm masking unit for masking the alarm issued from the performance monitor at least for a predetermined masking period from a starting time of a switching operation by the optical switch, and thus preventing the alarm from being issued.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an optical communication networksystem, and more particularly to an optical communication network systemfor performing switching of routes of an optical signal withoutconverting the optical signal into an electric signal.

[0002] In order to deal with a rapid increase in data trafficrepresented by the Internet and a sudden increase in demand formultimedia communications including images, audio and data, a higherspeed operation and a larger capacity operation have been pushed aheadfor a transmission line and a communication node, which constitute acommunication network, and there have been progresses made in theintroduction of an optical communication apparatus using an opticalfiber and an optical signal. In addition, in place of a conventionalcommunication apparatus for processing an optical signal, in which theoptical signal is converted into an electric signal once, studies havebeen conducted on practical use of an optical cross-connect apparatus(OXC) and an optical add-drop multiplexing apparatus (OADM) forperforming switching process such as switching of transmissionroutes/signal lines without converting an optical signal into anelectric signal. The OXC and the OADM uses an optical switch as a maincomponent for switching optical transmission lines. As an opticalswitch, various types have been known, e.g., a mechanical opticalswitch, an optical switch using a thermooptical effect, an opticalswitch using an electrooptical effect and the like. Among these types,the mechanical optical switch is most often used because a power lossthereof is the smallest.

[0003] For the practical use of the OXC or the OADM, it is essential toprovide an apparatus, which is configured to improve basic performancesuch as suppression of a power loss of an optical signal or the like andto be capable of properly switching and operating signal routes (or asystem itself), and to be excellent in reliability, availability andserviceability (hereinafter referred to as “RAS function”). In aconventional transmitter or a digital switching device such as amultiplexer processing an electric signal, performance of a signal to beprocessed has been monitored in a proper position, or a redundantconfiguration (e.g., duplication) in a part of an apparatus has beenadopted. Thus, an apparatus having an excellent RAS function has beenprovided.

[0004] In the conventional case of using the electric signal, timemultiplexing can be carried out up to 10 Gbps and, in principle, routescan be switched by using this technology. However, in a digitaltransmission system such as SONET/SDH, to execute process of high-levelcontrol management signal, 64 signals of 155 Mbps are arrayed inparallel development, and routes are switched. For such a speed, atechnology for switching by using a data buffering technology withoutany momentary power failures power-interruption has been known.

[0005] As described above, the OXC and the OADM uses the optical switchas the main component for switching routes of an optical signal.However, in the OXC and the OADM directly processing the optical signal,there occurs a problem, in the case of the most often used mechanicaloptical switch, that a switching speed is slow, which is severalmilli-seconds at the shortest, while a transmission rate of the opticalsignal to be passed is 10 Gbps or higher (e.g., 40 Gbps), which is muchhigher than that of an electric signal. Consequently, if switching ofsignal routes similar to the conventional apparatus for processing anelectric signal is simply executed for the OXC or the OADM, a momentarypower failure occurs, where an optical signal of several million bits,that is, several tens of frames, is lost because of its inability topass through the optical switch during optical signal route switching bythe optical switch. In other words, the momentary power failure that hasbeen prevented by the conventional apparatus for processing the electricsignal occurs in the OXC or the OADM directly processing the opticalsignal. Thus, a need arises to realize an optical signal switchingapparatus having an excellent RAS function on the assumption of presenceof a momentary power failure by an optical switch.

[0006] Generally, in the OXC or the OADM, in order to maintainperformance of an optical signal to be processed, after switching ofoptical routes, various factors are monitored, which include (1) opticalsignal power deterioration/failure [detection level: −20 dBm, detectiontime: order of 1 μsec.], (2) synchronous state of an operation clock[detection time: order of 1 μsec.], (3) synchronous state of an opticalsignal frame [detection time: 375 μsec,], (4) optical signal error rate(bit error rate, referred to as BER, hereinafter) [detection level:10⁻⁹, detection time: 10 sec.], and the like. This monitoring is carriedout for a predetermined time, and optical signal route (or systemitself) is properly switched to another when a trouble or a possibilityof a trouble is discovered. Such a trouble monitoring function isessential for an improvement of the RAS function. The detection levelsand the detection times, which are bracketed in the above-describedfactors, are only examples, and can be properly changed depending on aspeed of an optical signal to be processed by the apparatus or a size orinstalling place of the apparatus.

[0007] In the apparatus provided with the above-described troublemonitoring function, depending on an installing position of a monitoringcircuit or a monitoring method, a momentary power failure due to routeswitching by the optical switch may be detected as an optical signalpower failure, BER degradation or stepping-out of synchronization.Consequently, even if the switching is a normal operation, a situationmay occur where an alarm is given to the downstream side of an opticalsignal advancing direction or an apparatus for monitoring andcontrolling troubles. In addition, generally, the monitoring circuitalso verifies a normal state after completion of the route switching ormonitors recovery from the trouble. Thus, unless monitoring is carriedout by considering time necessary for route switching by the opticalswitch or an operation time of the above-described trouble monitoringfunction, even if the switching has been normally carried out, asituation may occur where an alarm is given to the downstream side ofthe optical signal advancing direction or the apparatus for monitoringand controlling troubles. In the OXC or the OADM, such a situationinduces repeating route switching even if an operation is normal.Consequently, an operation of the entire OXC or OADM, or an operation ofa communication system (network) using the OXC or the OADM becomesunstable, it brings about a state for the RAS function can not beoperated as desired. Needless to say, such a situation can be preventedby introducing a protective function for extending trouble detectiontime, recovery monitoring time and the like. However, such a method isnot preferable for an improvement of the RAS function because anoriginal alarm monitoring ability is reduced.

[0008] Meanwhile, in the conventional communication apparatus forprocessing an electric signal, such as a digital switching device andthe like, the one has been known, which is configured to previously maskerroneous information caused by an in-apparatus operation (e.g., systemswitching, hardware maintenance/switching) based on software instructionin order to prevent collection thereof and then to carry out anoperation, to collect by the software an alarm or management informationmonitored by hardware in the apparatus, and the like. However, themasking function by the software in the conventional communicationapparatus for processing an electric signal cannot simply be applied tothe OXC or the OADM.

[0009] As a specific example, when an optical route is switched by theoptical switch, a momentary power failure causes an optical signal powerfailure, and stepping-out of clock synchronization (hereinafter referredto as clock stepping-out) and stepping-out of frame synchronization(hereinafter referred to as frame stepping-out) of a transmissionsignal. However, recovery from the optical signal power failure isdetected during switching time (about 1 milli-sec.) after completion ofswitching. Meanwhile, for the clock stepping-out and the framestepping-out, after optical signal power is recovered by a newconnection, new clock and frame synchronization must be performed. Timenecessary for verifying re-synchronization exceeds 1 milli-sec. Further,10000 frames are necessary for BER measurement since framesynchronization is secured, and the process must wait for 10 sec.Consequently, when correct operation of route switching is carried outby distinguishing a momentary power failure due to switching of theoptical switch from a disconnection of an optical fiber as a fixedtrouble, if only the conventional trouble detection method or theconventional masking function by the software simply is applied to theOXC or the OADM, the RAS function becomes short. Thus, there is a demandfor an OXC or an OADM having an excellent RAS function for detecting areal trouble and switching routes in consideration of a combination of aplurality of factors for trouble detection and monitoring time thereofwith a trouble detection/recovery detection operation carried outfollowing disposition of a trouble detection circuit in an apparatus.Furthermore, there is a demand for an OXC or an OADM preventingnotification of an alarm to a downstream side of an optical signaladvancing direction or an apparatus for monitoring and controlling atrouble even if a momentary power failure occurs due to route switching,and preventing induced re-switching of routes while an operation isnormal.

SUMMARY OF THE INVENTION

[0010] An object of the present invention is to provide a highlyreliable optical communication network system for switching routeswithout any conversion of an optical signal, which is preventingnotification of an erroneous alarm during switching by an opticalswitch.

[0011] In order to achieve the above-described object, according to thepresent invention, a configuration is adopted, where an alarm-issuedfrom a monitor for monitoring performance of an optical signal having aroute set by an optical switch is masked for a predetermined maskingperiod from a starting time of a switching operation by the opticalswitch. Thus, even if an alarm is issued because of a change inperformance of an optical signal by a normal switching operation of theoptical switch, it is possible to prevent the alarm from beingrecognized as such by the system.

[0012] The monitor can be adapted to detect the performance of theoptical signal regarding a plurality of predetermined factors, and toissue an alarm for each of the plurality of factors. In this case, theabove-described masking period should preferably be set for each of theplurality of factors. Accordingly, it is possible to mask an alarm foreach of the plurality of factors and each of the detected factors ofoptical signal performance only for a minimum necessary period, whileoptical signal performance is reduced because of the normal switchingoperation of the optical switch.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Preferred embodiments of the present invention will now bedescribed in conjunction with the accompanying drawings, in which:

[0014]FIG. 1 is an explanatory view showing an entire configuration ofan optical communication network system according to an embodiment ofthe present invention;

[0015]FIG. 2 is a block diagram showing a basic configuration of anoptical route setting apparatus 100 used for the optical communicationnetwork system of the embodiment of the present invention;

[0016]FIG. 3 is a flowchart showing a process for switching of opticalroutes in the optical route setting apparatus 100 of FIG. 2;

[0017] FIGS. 4(a) to (h) are explanatory views showing states ofrespective portions and signal time charts when an optical routeswitching operation is normally carried out in the optical route settingapparatus 100 of FIG. 2;

[0018] FIGS. 5(a) to (h) are explanatory views showing states of therespective portions and signal time charts when an optical power failurealarm is issued after an optical route switching operation in theoptical route setting apparatus 100 of FIG. 2;

[0019] FIGS. 6(a) to (h) are explanatory views showing states of therespective portions and signal time charts when an error rate alarm isissued after the optical route switching operation in the optical routesetting apparatus 100 of FIG. 2;

[0020]FIG. 7 is a block diagram showing a specific configuration of theoptical route setting apparatus 100 used for the optical communicationnetwork system of the embodiment of the present invention;

[0021]FIG. 8 is an explanatory view showing a state of each bit area ofan alarm register 352 and a mask register 363 in a state where aswitching operation of the optical route setting apparatus 100 of FIG. 7is not carried out, and no alarms are issued;

[0022]FIG. 9 is an explanatory view showing a masking period and anoutput after masking for each alarm, which are stored in an alarmmanagement memory 344 of the optical route setting apparatus 100 of FIG.7;

[0023]FIG. 10 is an explanatory view showing states 1 to 5 of bit areasof failure alarms and error rate alarms of the alarm register 352 andthe mask register 353 of the optical route setting apparatus 100 of FIG.7;

[0024]FIG. 11 is an explanatory view showing an optical fiber 2006partially disposed double to solve a trouble in a ring of the opticalfiber 2006 of the optical communication network system of FIG. 1;

[0025]FIG. 12 is a block diagram showing a configuration of an opticaladd-drop multiplexing apparatus (OADM) of the optical communicationnetwork system of FIG. 1;

[0026]FIG. 13 is a block diagram showing a configuration of an opticalcross-connect apparatus (OXC) of the optical communication networksystem of FIG. 1;

[0027]FIG. 14 is a block diagram showing a configuration of the opticalroute setting apparatus 100 of the embodiment of the present invention,where an alarm mask is achieved by software;

[0028]FIG. 15 is a flowchart showing an operation of a CPU 342 of theoptical route setting apparatus 100 of FIG. 14;

[0029]FIG. 16 is a flowchart showing an operation of the CPU 342 of theoptical route setting apparatus 100 of FIG. 14; and

[0030]FIG. 17 is a flowchart showing an operation of the CPU 342 of theoptical route setting apparatus 100 of FIG. 14.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] Next, description will be made for an optical communicationnetwork system according to an embodiment of the present invention.

[0032] As shown in FIG. 1, an optical communication network system ofthis embodiment includes optical add-drop multiplexing apparatuses(OADM) 1003 to 1009 and optical cross-connect apparatuses (OXC) 1001 and1002, which are connected through optical fibers 2001 to 2006.Specifically, the optical add-drop multiplexing apparatuses (OADM) 1003to 1005 are connected in a ring shape through the optical fiber 2005,and optical add-drop multiplexing apparatuses (OADM) 1006 to 1009 areconnected in a ring shape through the optical fiber 2006. The opticalfiber 2005 and the optical fibers 2001 to 2004 are connected by theoptical cross-connect apparatus (OXC) 1001. The optical cross-connectapparatus (OXC) 1001 is also connected to the optical add-dropmultiplexing apparatus (OADM) 1006. The optical fiber 2003 also areconnected to other optical fibers through the optical cross-connectapparatus (OXC) 1002.

[0033] As shown in FIG. 12, the optical add-drop multiplexing apparatus(OADM) 1003 includes a divider 121 for division (demultiplexing) anoptical signal having been subjected to wavelength-multiplexing, anoptical route setting apparatus (an optical switching apparatus) 100 forswitching routes of an optical signal, and a multiplexer 122 formultiplexing optical signals. Thus, a multiplexed optical signalreceived through the optical fiber 2005 is divided by the wavelengthdivider 121, and a necessary optical signal is taken out by the opticalroute setting apparatus 100 and outputted through the optical fiber 2007to an external apparatus. The optical signal received from the externalapparatus through the optical fiber 2007 is multiplexed with the otheroptical signal by a wavelength multiplexer 122 and sent to the opticalfiber 2005. The optical add-drop multiplexing apparatuses (OADM) 1004 to1009 are similar in configuration to the optical add-drop multiplexingapparatus (OADM) 1003. That is, a necessary optical signal is taken outfrom multiplexed optical signals received through the optical fibers2005 and 2006, and outputted through optical fibers 2008 to 2013 to theexternal apparatus. The optical signal received from the externalapparatus is multiplexed with the other optical signal, and then sent tothe optical fibers 2005 and 2006.

[0034] Meanwhile, as shown in FIG. 13, the optical cross-connectapparatus (OXC) 1001 includes divider 131, 132 and the like for division(demultiplexing) an optical signal having been subjected towavelength-multiplexing, an optical route setting apparatus 100 forswitching routes of an optical signal, and multiplexers 133, 134 and thelike for multiplexing an optical signal. Thus, a multiplexed opticalsignal received through the optical fibers 2001, 2005 and the like isdivided by the divider 131, 132 and the like, and a route is switched bythe optical route setting apparatus 100 to an optical fiber, whichbecomes a destination for each optical signal. Then, the optical signalis multiplexed again by the multiplexers 133, 134 and the like, andoutputted to the optical fibers 2005, 2004 and the like. In addition,the optical cross-connect apparatus (OXC) 1001 transmits and receiversan optical signal locally with the optical add-drop multiplexingapparatus (OADM) 1006 through the optical fiber 2010. The opticalcross-connect apparatus (OXC) 1002 is also similar in configuration tothe optical cross-connect apparatus (OXC) 1001.

[0035] As described above, each of the optical add-drop multiplexingapparatuses (OADM) 1003 to 1009 and the optical cross-connectapparatuses (OXC) 1001 and 1002 include the optical route settingapparatus 100 for switching routes without converting an optical signalinto an electric signal. The optical route setting apparatus 100includes an optical switch 300 for switching routes of an opticalsignal, a control unit 305 for controlling an operation of the opticalswitch 300, and optical performance monitors 310. The opticalperformance monitor 310 detects four factors: power deterioration of anoptical signal passed through the optical switch 300; a synchronousstate of an operation clock; a synchronous state of an optical signalframe; and an error rate of an optical signal. A result of the detectionby the optical performance monitor 310 is supplied to the control unit305.

[0036] The optical communication network system of the embodiment has astructure for recovery from a trouble in the case of occurrence thereof.Specifically, a transmission line is constructed by connecting theoptical add-drop multiplexing apparatuses (OADM) 1006 to 1009 in a ringshape through the optical fiber 2006. As shown in FIG. 11, in thistransmission line the optical fibers 2006-4 connecting between theoptical add-drop multiplexing apparatuses (OADM) 1006 and 1007 areconstituted of two parallel optical fibers 2006-4 a and 2006-4 b, andthe optical add-drop multiplexing apparatuses (OADM) 1006 and 1007 canselect any one of the optical fibers 2006-4 a and 2006-4 b by aswitching operation of the optical route setting apparatus 100 totransmit an optical signal. In the transmission line of FIG. 11,normally, a communication route for transmitting an optical signal fromthe optical fiber 2010 to the optical fiber 2012 is preset to passthrough the optical add-drop multiplexing apparatus (OADM) 1006, theoptical fiber 2006-4 a, the optical add-drop multiplexing apparatus(OADM) 1007, the optical fiber 2006-3, the optical add-drop multiplexingapparatus (OADM) 1008, and the optical fiber 2012. It is also arrangedbeforehand that a trouble occurring in the optical fiber 2006-4 a issolved by switching to the optical fiber 2006-4 b to transmit theoptical signal. It is further arranged beforehand that troublesoccurring in both of the optical fibers 2006-4 a and 2006-4 b are solvedby switching the transmission line to pass through the optical add-dropmultiplexing apparatus (OADM) 1006, the optical fiber 2006-1, theoptical add-drop multiplexing apparatus (OADM) 1009, the optical fiber2006-2, the optical add-drop multiplexing apparatus (OADM) 1008, and theoptical fiber 2012. These switching operations for recovery fromtroubles are carried out under a supervisory and control system (OpS),not shown, which instruct route switching to the control unit 305 of theoptical route setting apparatus 100 of each of the optical add-dropmultiplexing apparatuses (OADM) 1006 to 1009, or under self-judgment ofthe control unit 305.

[0037] In the embodiment, a mechanical optical switch is used for theoptical switch 300 of the optical route setting apparatus 100. Thisoptical switch 300 includes optical fibers disposed with end surfacesfacing each other, and a driver for mechanically shifting one of theoptical fibers vertically. By moving the optical fiber with the driver,a positional relation is set, where the optical switch faces endsurfaces of optical fibers disposed adjacently to each other, andcarries out switching. Such a mechanical optical switch requires severalmilli-sec. at the shortest from a start of a switching operation to anend thereof and, during this period, a momentary power failure occurs,where the optical signal cannot be passed through the optical switch 300and lost. In addition, during the switching operation, stepping-out ofan operation clock synchronization of a signal and stepping-out of anoptical signal frame synchronization thereof occur. Thus, even when aset route of the optical route setting apparatus 100 is changed withoutany troubles or the like, during the switching operation of the opticalswitch 300, a momentary power failure or stepping-out is detected by theoptical performance monitor 310. When a trouble is recognized while theswitching operation is actually normal, a recovery operation from thetrouble is started as described above with reference to FIG. 11. Thus,according to this embodiment, the optical route setting apparatus 100 isconstructed in the following manner, and an optical communicationnetwork system excellent in reliability, availability and serviceabilityis provided by using the optical switch 300.

[0038] By properly selecting components, the optical route settingapparatus 100 of the embodiment can readily construct a flexiblecommunication network capable of dealing with various transmission ratesand multiplexing degrees of optical signals. For example, an opticalsignal or the like having a transmission rate of STM-0 (51.84 MHz) setby ITU-T Recommendation can be used, and there are no limitations onpresence of wavelength-division-multiplexing or the number thereof.

[0039] First, description will be made for features of the optical routesetting apparatus of this embodiment with reference to a simplifiedconfiguration of FIG. 2. In the simplified optical route settingapparatus 100 of FIG. 2, attention is paid to a switching operation ofone of the optical switches N×N or N×M of FIG. 12 or 13, and two inputsignals 1 and 2, and one output signal are shown. The input signals 1and 2 are respectively inputted from routes 201 and 202 to the opticalswitch 300, and one selected by the optical switch 300 is outputted froma route 203. The optical signal outputted from the route 203 is passedthrough the optical performance monitor 310, and is detected about fourfactors, i.e., power deterioration of the optical signal, a synchronousstate of an operation clock, a synchronous state of an optical signalframe and an error rate of the optical signal. Then, if each factor islower than a predetermined value, an alarm is issued for each of thefour factors. The control unit 305 includes a system control unit 302,an alarm mask 303, and a timer 304. The system control unit 302 controlsroute switching of the optical switch 300, and collects alarminformation from the optical performance monitor 310 through the alarmmask 303. The alarm mask 303 includes an optical power failure alarmmask 303 a, an operation clock synchronization stepping-out alarm mask303 b (herein after referred to as “operation clock stepping-out alarmmask 303 b”), a frame synchronization stepping-out alarm mask 303 c(herein after referred to as “operation clock stepping-out alarm mask303 c”) and an error rate alarm mask 303 d corresponding to factors, forwhich the optical performance monitor 310 issues alarms during opticalroute switching.

[0040] Hereinafter, description will be made for an optical routeswitching operation of the optical route setting apparatus 100 byreferring to the flowchart of FIG. 3. After an input of a routeswitching request from the supervisory and control system (OpS) or thelike, not shown, to the system control unit 302 (step 400), the systemcontrol unit 302 actuates a timer 304 by a trigger signal and sets thealarm mask 303 in a masking state (step 420). The system control unit302 has a built-in memory, in which masking periods of the optical powerfailure alarm mask 303 a, the operation clock stepping-out alarm mask303 b, the frame stepping-out alarm mask 303 c and the error rate alarmmask 303 d are prestored. The system control unit 302 reads each storedmasking period, and sets the masking period in the timer 304corresponding to an alarm of each of the four factors. In thisembodiment, the masking periods of the optical power failure alarm mask303 a, the operation clock stepping-out alarm mask 303 b and the framestepping-out alarm mask 303 c are set equal to 10 ms, and the maskingperiod of the error rate alarm mask 303 d is set equal to 15 s. In thecase of a normal switching operation, these masking periods are presetto individual periods necessary until a normal state is recovered fromthe alarm state.

[0041] After the setting of the alarm mask 303, the system control unit302 sends a switching command signal to the driver of the optical switch300, and the driver switches optical routes (step 480). Immediatelyafter the switching of the optical routes, a failure occurs in theoptical signal, operation clock stepping-out and optical signal framestepping-out occur, and an error rate cannot be correctly measured.Therefore, alarms are issued from the optical performance monitor 310.However, since these have been masked respectively by the alarm masks303 a, 303 b, 303 c and 303 d, the system control unit 302 recognizes noalarms. After completion of the optical route switching, and passage ofthe set masking period, the timer 304 outputs mask releasing signalsrespectively to the above-described four alarm masks 303 a, 303 b, 303 cand 303 d (step 450), and releases the masks (step 460). When all thealarm masks 303 a, 303 b, 303 c and 303 d are released (step 470), theprocess returns to a normal alarm monitoring state for the four alarms(step 490).

[0042] Now, description will be made for an operation of the opticalroute setting apparatus 100 by referring to time charts of FIGS. 4(a) to(h), FIGS. 5(a) to (h), and FIGS. 6(a) to (h).

[0043] First, by using the time charts of FIGS. 4(a) to (h), descriptionis made for a case where the optical switch 300 of the optical routesetting apparatus of FIG. 2 is normally switched.

[0044] Here, it is assumed that the optical switch 300 switches a signaloutputted from the output route 203 from an optical signal 1 to anoptical signal 2. A state changed with time is shown in FIG. 4(h). Astate before switching is a “STATE 1” 511 of FIG. 4(h). When there is aswitching request, the system control unit 302 executes the operation ofstep 420 to set masks in the optical power failure alarm mask 303 a, theoperation clock stepping-out alarm mask 303 b, the frame stepping-outalarm mask 303 c and the error rate alarm mask 303 d. Accordingly, a“STATE 2” 512 of FIG. 4(h) is set. Here, states of only an optical powerfailure alarm mask 504 and an error rate alarm mask 505 are shown inFIGS. 4(e) and (f).

[0045] The system control unit 302 executes the operation of step 430 tosend a switching command signal (FIG. 4(a)) to the driver of the opticalswitch 300. In the optical switch 300 operates the driver operates tochange mechanically a connection state, realizing a change fromconnection 1 to connection 2, and a selected signal (FIG. 4(c)) of theoptical switch 300 is switched from the optical signal 1 to the opticalsignal 2. Time necessary for this switching operation of the opticalswitch 300 is about several ms, and a state executing this switchingoperation is a “STATE 3” 513 of FIG. 4(h).

[0046] In the “STATE 3” 513, the optical signal cannot be passed throughthe optical switch 300. Thus, a power failure of the optical signaloccurs, and an optical power failure alarm is issued from the alarmmonitor 310. Moreover, since the error rate cannot be measured due tothe optical power failure, the error rate alarm is also issued. In this“STATE 3” 513, since masks are set in the optical power failure alarmmask 303 a and the error rate alarm mask 303 d as shown in FIG. 4(e) and(f), no alarm signals are outputted from the alarm masks 303 a and 303 dto the system control unit 302 as shown in FIG. 4(g), and the systemcontrol unit 302 recognizes a state as normal. After completion of theswitching operation of the optical switch 300, the optical power failurealarm is released because of recovery of the optical power. Then, theoptical power failure alarm mask 303 a is released by the operations ofsteps 450 and 460, a “STATE 4” 514 is set. Though not shown, in the“STATE 3” 513, operation clock stepping-out and frame stepping-out occurfollowing the switching operation, and alarms are issued from theoptical performance monitor 310. However, since there are masks set inthe operation clock stepping-out alarm mask 303 b and the framestepping-out alarm mask 303 c, no alarm signals are outputted to thesystem control unit 302. During this masking period, the opticalperformance monitor 310 re-takes operation clock synchronization andframe synchronization, and releases the alarms. In addition, the opticalperformance monitor 310 starts measurement of an error rate in matchingwith the optical power recovery. The error rate measurement takes time,an error rate alarm is continued, and a normal state is recovered aftera passage of certain time. Then, the error rate alarm mask 505 has itsmask released after a passage of time required for measurement (steps450 and 460), and a “STATE 5” 515 is set.

[0047] As a result, no alarms are issued from the alarm mask 303 to thesystem control unit 302 during switching to a normal signal, alwayssetting a normal state.

[0048] Next, description will be made for a case where, after theswitching operation of the optical switch 300 of the optical routesetting apparatus of FIG. 2, an optical signal power failure state isset by referring to time charts of FIGS. 5(a) to (h).

[0049] Causes of an optical signal power failure state after switchingare, for example, a case where an intensity itself of the optical signal2 inputted to the optical switch 300 is weak, a case where the switchingoperation of the optical switch 300 is not carried out normally, and thelike. In this case, until a “STATE 2” 512 of FIG. 5(h) is similar to the“STATE 2” 512 of FIGS. 4(a) to (h). In a “STATE 3” 513, an opticalsignal power failure occurs by the switching operation of the opticalswitch 300, and an optical power failure alarm and an error rate alarmare issued from the optical performance monitor 310. The optical powerfailure alarm mask 303 a is masked until a “STATE 3” 513 where theswitching of the optical switch 300 is completed. In a normal casesimilar to that shown in FIG. 4(d), the optical power failure alarm isreleased before an end of the “STATE 3” 513. However, if the opticalsignal 2 is abnormal and its intensity is weak as shown in FIG. 5(c),the optical power failure alarm of the optical performance monitor 310continues even after the mask of the optical power failure alarm mask303 a is released. Thus, at a point of time when a “STATE 4” 514 is set,an optical power failure alarm is issued from the optical power alarmmask 303 a to the system control unit 302, and the system control unit302 recognizes the failure alarm. In this case, since the optical powerfailure continues, an error rate alarm also continues. From a point oftime when a “STATE 5” 515 is set, an error rate alarm is issued from theerror rate alarm mask 303 d to the system control unit 302, and thesystem control unit 302 recognizes the error rate alarm. Accordingly,the system unit 302 notifies an occurrence of a trouble to thesupervisory and control system (OpS) and, under instruction of thesupervisory and control system (OpS), a new route switching operation isstarted to make recovery from the trouble shown in FIG. 11.

[0050] Next, description will be made for a case where an error rate ofan optical signal is reduced after the switching operation carried outby the optical switch 300 of the optical route setting apparatus of FIG.2, by referring to the time charts of FIGS. 6(a) to (h).

[0051] Causes of deteriorations in error rates after switching are, forexample, a case where there is much noise in an optical signal 2 itselfinputted to the optical switch 300, a case where interference occurs inthe optical switch 300 because of the switching operation of the opticalswitch 300. In this case, a state until a “STATE 4” 514 of FIG. 6(h) issimilar to a state until the “STATE 4” 514 of FIGS. 4(a) to (h). In thecase of normal switching, as shown in FIG. 4(d), until the period of the“STATE 4” 514, the error rate alarm is released. However, in the case ofFIG. 6(d), since the error rate is reduced in the optical signal 2, theerror rate alarm continues even when a “STATE 5” 515 is set, where theerror rate alarm mask 303 d is released in FIG. 6(f). Accordingly, inthe “STATE 5” 515, an error rate alarm is issued from the error ratealarm mask 303 d to the system control unit 302, and the system controlunit 302 then recognizes the error rate alarm. Thus, the system controlunit 302 notifies an occurrence of a trouble to the supervisory andcontrol system (OpS) and, under instruction of the supervisory controlsystem (OpS), a new route switching operation is started to makerecovery from the trouble, as shown in FIG. 11.

[0052] Next, description will be made for a specific configuration ofthe optical route setting apparatus 100 of the embodiment by referringto FIG. 7. This optical route setting apparatus 100 monitors opticalperformance in an optical synchronous network (SONET) of 10 Gbit/s, anduses an optical switch 300 of switching time 1 ms. The optical routesetting apparatus 100 includes N×N optical switches 300, an opticalperformance monitor 310 disposed in each of output routes, amounting toN in number, of the optical switches 300, and a control unit 305.

[0053] The optical performance monitor 310 includes an optical splitter361 for splitting an output of the optical switch 300, a power monitor362 for detecting whether or not split optical power is equal to apredetermined value or higher, a photoelectric converter 363 forconverting an optical signal outputted from the optical switch 300 intoan electric signal, a performance monitoring circuit 364 for evaluatingclock synchronization, frame synchronization and a bit error rateregarding the electric signal obtained by the conversion, and anelectrooptical signal converter 365 for re-converting the electricsignal into an optical signal. The power monitor 362 issues an opticalpower failure alarm when power of an optical signal is lower than apredetermined value. The performance monitoring circuit 364 monitorssynchronous states of a reference clock outputted from a built-inreference clock circuit with clock and frame signals extracted from areceived electric signal, outputs an operation clock stepping-out alarmand a frame stepping-out alarm when stepping-out occurs, takessynchronization again within a fixed time, and stops the alarms whensynchronization is established. In addition, the performance monitoringcircuit 364 detects a bit error rate of an electric signal, and issuesan error rate alarm when the error rate is lowered than a predeterminedvalue.

[0054] The control unit 305 includes an alarm masking unit 303, a timer346, and a portion equivalent to the system control unit 302 of FIG. 2.In the configuration of FIG. 7, the portion equivalent to the systemcontrol unit 302 includes a CPU 342, a switching information memory 343,an alarm management memory 344, a switching control circuit 345 and anI/O unit 341.

[0055] The alarm masking unit 303 includes an alarm interface circuit354 for receiving four kinds of alarms from the optical performancemonitors 310 amounting to N in number, an alarm register 352 for storingthe alarms, a mask register 353 for setting an alarm mask, and an alarmissuing unit 351. As shown in FIG. 8, corresponding to output ports(routes), amounting to N in number, of the optical switch 300, the alarmregister 352 has areas (bit areas) for writing “0” (normal) or “1”(there is an alarm), which indicate whether or not the opticalperformance monitor 310 issues a failure alarm, an error rate alarm, anoperation clock stepping-out alarm and a frame stepping-out alarm.Similarly, corresponding to the output ports (routes), amounting to N innumber, of the optical switch 300, the mask register 353 has areas (bitareas) for writing “0” (mask is set)” or “1” (mask is released)indicating whether or not masks are respectively set in a failure alarm,an error rate alarm, an operation clock stepping-out alarm and a framestepping-out alarm. The alarm issuing unit 351 of the mask register 353obtains a product of “0” or “1” written in the corresponding areas ofthe alarm register 352 and the mask register 353, and outputs the alarmof its output port, if “1”, to the CPU 342.

[0056] Writing of “0” or “1” in the alarm register 352 is carried out bythe alarm interface circuit 354. Writing of “0” or “1” in the maskregister 353 is carried out by the CPU 342, which refers to a maskingperiod of each alarm prestored in the alarm management memory 344 asshown in FIG. 9, and operates the timer 346. In the switchinginformation memory 343, a switching state of the optical switch 300 isstored. The masking period of each alarm prestored in the alarmmanagement memory 344 has been decided based on the use of opticalperformance monitoring in the optical synchronous network (SONET) of 10Gbit/s and an optical switch of switching time 1 ms. In this case, asshown in FIG. 9, the masking periods are set at 10 ms for a failurealarm, an operation clock stepping-out alarm and a frame stepping-outalarm, and at 15 s for an error rate alarm.

[0057] Now, description will be made concretely for an operation of thecontrol unit 305 of the optical route setting apparatus 100 of FIG. 7.

[0058] First, description will be made for a state where an input port201-1 is connected to an output port 203-N by the optical switch 300 andnormally operated (no switching operations are carried out). In the maskregister 353, as shown in FIG. 8, “1” is written, indicating that allmasks are released. Since there are no alarms issued from the opticalperformance monitor 310, a value of the alarm register 352 is “0”indicating that all are normal. This case is a “STATE 1” 611 shown inFIG. 10. Thus, no alarms are issued from the alarm issuing unit 351 tothe CPU 342. FIG. 10 shows only a failure alarm and an error rate alarmas kinds of alarms, and an operation clock stepping-out alarm and aframe stepping-out alarm are not shown. In this case, the timer 346 isreset.

[0059] When a bit error trouble occurs in an optical signal of the inputport 201-1, a degradation in a bit error rate is detected by theperformance monitoring circuit 364 in the optical performance monitor360 of the output port 203-N, and an alarm is issued. This alarm isreceived by the alarm interface circuit 354, and “1” indicating presenceof an alarm is written in a bit area corresponding to a bit error rateof a port N of the alarm register 352. In this case, since 1 indicatingmask releasing is set in the mask register 358, an alarm is issued fromthe alarm issuing unit 351, and then received by the CPU 342.Determining that a trouble has occurred, the CPU 342 notifies thetrouble to the supervisory and control system (OpS) and, underinstruction of the supervisory and control system (OpS), a routeswitching operation is started for recovery from the trouble shown inFIG. 11.

[0060] Next, description will be made for an operation when the CPU 342receives, through the I/O unit 341, instruction to switch the input portconnected to the output port 203-N of the optical switch 300 from theinput port 201-1 to the input port 201-N.

[0061] Before issuing a switching command of the optical switch 300, theCPU 342 sets the alarm mask 303 and starts the timer 346. First, “0”indicating mask setting is written in each of bit areas corresponding toan optical power failure alarm, an error rate alarm, an operation clockstepping-out alarm and a frame stepping-out alarm of the output port203-N of the mask register 353 of the alarm mask 303. Then, a maskingperiod of each alarm is read from the alarm management memory 344, setin the timer 346, and the timer 346 is started. Accordingly, a “STATE 2”612 of FIG. 10 is set. In response to a switching request received fromthe I/O unit 341, the CPU 342 calculates a new switching state from theswitching information memory 343, and outputs a switching command fromthe switching control unit 345 to the optical switch 300. The opticalswitch 300 operates the driver to connect the input port 201-N to theoutput port 203-N according to the switching command. The CPU 342 storesa new state in the switching information memory 343.

[0062] By the switching operation of the optical switch 300, an opticalpower failure occurs in an output optical signal of the output port203-N. The power monitor 362 of the optical performance monitor 310 ofthe output port 203-N detects the optical power failure, and issues anoptical power failure alarm. The optical power failure alarm is receivedby the alarm interface circuit 354, and the alarm interface circuit 354sets a value of a bit area corresponding to the optical power failurealarm of the port N of the alarm register 352 to “1” indicating presenceof an alarm. The performance monitoring circuit 364 detects an errorrate degradation, operation clock stepping-out and frame stepping-out,and issues respective alarms. These alarms are received by the alarminterface circuit 354, and the bit areas of the error rate alarm, theoperation clock stepping-out alarm and the frame stepping-out alarm ofthe port N of the alarm register 352 are set to “1” indicating presenceof an alarm. In this case, as described above, in the mask register 353,“0” indicating masking is set in the bit area corresponding to eachalarm of the port N. Accordingly, a result of obtaining a logicalproduct of the alarm register 362 and the mask register 353 by the alarmissuing unit 351 is “0”, and thus the alarm issuing unit 351 issues noalarms. This is a “STATE 3” 613 of FIG. 10. In the case of an opticalpower failure, since an error rate cannot be measured, the performancemonitoring circuit 364 stops error rate measurement until optical poweris verified, and resumes the error rate measurement after recovery ofthe optical power.

[0063] Switching by the optical switch 300 is completed after about 1 msfrom the reception of the switching command, and the optical signalreaches the output port 203-N. Since the optical signal also reaches thepower monitor 362, the detection of the optical power failure alarm isreleased and, by the alarm interface circuit 354, “0” indicating anormal state is written in the bit area of the optical power failurealarm of the port N of the alarm register 352. After 10 ms from theswitching, optical power failure alarm mask releasing time is notifiedfrom the timer 353 to the CPU 342, and “1” indicating mask releasing iswritten in the optical power failure bit area of the mask register 353.In this case, since the optical power failure alarm has been released,the alarm issuing unit 351 gives no alarms to the CPU 342. This is a“STATE 4” 614 of FIG. 10.

[0064] The performance monitoring circuit 364 of the optical performancemonitor 310 of the output port 203-N resumes the error rate measurementafter recovery from the optical power failure and, with a passage of apredetermined time (about 10 sec.), an error rate of 10⁻⁹ or lower canbe measured. Accordingly, the error rate alarm is released and, throughthe alarm interface circuit 354, “0” indicating a normal state iswritten in the bit area of the error rate alarm of the port N of thealarm register 352.

[0065] After 15 sec., the error rate alarm releasing time is notifiedfrom the timer 346 to the CPU 342, and “1” indicating mask releasing iswritten in the bit area of the error rate alarm of the port N of themask register 353. In this case, since the error rate alarm has beenreleased, and “0” has been written in the corresponding bit area of thealarm register, the alarm issuing unit 361 gives no alarms to the CPU342. After all the masks are released, the process returns to a normalstate “STATE 5” 615.

[0066] If the corresponding bit area of the alarm resister 352 is “1”indicating an abnormal state even when a mask releasing time specifiedby the timer 346 is reached, and “1” indicating mask releasing iswritten in the mask register 353, the alarm issuing unit 351 issues analarm to the CPU 342. Determining that a trouble has occurred, the CPU342 notifies the trouble to the supervisory and control system (OpS)and, under instruction of the supervisory and control system (OpS), anew route switching operation is started for recovery from the troubleshown in FIG. 11, or automatic switching is carried out forself-recovery.

[0067] As described above, the optical route setting apparatus 100 ofthe embodiment executes alarm masking for an optical signal powerfailure, an error rate degradation, operation clock stepping-out andframe stepping-out, which occur following the normal switching operationof the optical switch 300, and thus erroneous recognition of aoccurrence of troubles can be prevented. That is, alarms received fromthe performance monitor 310 are distinguished between (1) one caused bya trouble and (2) one caused by a route switching operation of theoptical switch 300, and issuance of an alarm to the other connectedapparatus is controlled. Specifically, in the case of (2), an alarm isprevented from being issued to the other optical add-drop multiplexingapparatus (OADM) 1003, the optical cross-connect apparatus (OXC) 1001 orthe supervisory and control system (OpS) (not shown), which is connectedthrough the optical fibers 2005, 2006 or the like to the self apparatus.Thus, there is no possibility of starting a new route switchingoperation or the like for recovery from a trouble despite of the normalswitching operation of the optical switch 300. Therefore, it is possibleto provide a highly reliable optical route setting apparatus.

[0068] In addition, in the optical route setting apparatus 100 of theembodiment, proper alarm masking periods can be set for the plurality offactors, i.e., an optical signal power failure, an error ratedegradation, operation clock stepping-out and frame stepping-out.Accordingly, without reducing detection accuracy of an occurrence ofreal troubles, erroneous recognition of an alarm following the normalswitching operation of the optical switch can be prevented, thusenhancing reliability.

[0069] Therefore, by using the optical add-drop multiplexing apparatus(OADM) and the optical cross-connect apparatus (OXC) for the opticalroute setting apparatus 100 of the embodiment, it is possible to providean optical add-drop multiplexing apparatus (OADM), an opticalcross-connect apparatus (OXC) and an optical communication networksystem, which are all excellent in reliability, availability andserviceability.

[0070] In the optical route setting apparatus 100 shown in FIG. 7, thealarm mask 303 is constructed in such a manner that, by using the alarmregister 352 and the mask register 353, the alarm issuing unit 351obtains a logical product of bits (“1” or “0”) stored in thecorresponding areas of both registers, and then issues an alarm.However, by using the CPU 342 to execute a program, alarm processingsetting an alarm mask can be executed. This processing is now describedby referring to FIGS. 14 to 17. In this case, as shown in FIG. 14, theoptical route setting apparatus 100 includes no constituent element ofalarm masks 303. In the alarm management memory 344, in addition to themasking period of each alarm of FIG. 9, programs as shown in FIGS. 15 to17 are prestored. The CPU 342 performs alarm processing and switching ofthe optical switch 300 by reading and executing the programs of thealarm management memory 344. The CPU 342 initializes the programs uponreading in step 151 of FIG. 15, inputs a loop, and executes alarmprocessing (step 152) unless there is a route switching request (step153) from the unillustrated supervisory and control system (OpS) orresetting (step 155). The alarm processing is finished without anychanges if the CPU 342 has received no alarms from the alarm interfacecircuit 353 (step 161) as shown in FIG. 16. If the CPU 342 has receivedan alarm, the CPU 342 itself determines a necessity of processing forrecovery from a trouble. If the processing for recovery is necessary,then the CPU 342 performs an operation for switching the optical switch300 to a predetermined route (steps 162 and 163). When necessary, forexample when a trouble cannot be solved by a switching operation in theself optical route setting apparatus 100, notification (steps 164 and165) is made to the not-shown supervisory and control system (OpS), andthe process is finished. In this case, the process proceeds to step 153of FIG. 15, a proper switching request is received from the not-shownsupervisory and control system (OpS), and a switching operation iscarried out (step 154).

[0071] In a case where a route switching request is received from theexternal apparatus, the switching operation (step 154) or the switchingoperation for recovery (step 163) is carried out in a manner shown inFIG. 17. First, the CPU 342 reads a masking period of each alarm fromthe alarm management memory 344 for each output port, starts masking,and actuates the timer 346. In this case, a configuration can be made insuch a manner that an area is previously provided in the alarmmanagement memory 344 to write-in presetting of an alarm mask for eachoutput port, and the CPU 342 sets a mask for each writing of “0” or “1”in this area. In a mask set state, a switching command is outputted tothe optical switch 300 (step 173). Accordingly, the optical switch 300executes route switching. The CPU 342 proceeds to step 152, and executesa flow of alarm processing 152 shown in FIG. 16. However, if the CPU 342receives an alarm from the alarm interface circuit 353 in step 161,during the alarm masking period of the port in step 172, processing isexecuted determining that there are no alarms. The CPU 342 receives maskreleasing time information from the timer, and releases a specified maskwhen the mask releasing time is reached (steps 175 and 176). Forexample, the mask is released by writing “1” indicating mask releasingin the alarm mask writing area of the alarm management memory. Regardingthe alarm of the mask-released port, if the CPU 342 receives an alarmfrom the alarm interface circuit 353 in the alarm processing of step152, the process proceeds to step 162 in the flow of FIG. 16, andrecovery is made when necessary. When the masking time of a last alarmmask reaches a releasing time (step 175), alarm processing is executed(step 152), all the masks are released (step 174), and a result of theswitching is written in the switching information memory 343 (step 177).Thus, the switching operation of the optical routes is finished

[0072] As described above, according to the configuration of FIGS. 14 to17, since the alarm mask can be realized by software, the optical routeswitching apparatus 100 of the embodiment can be provided with a simpleconfiguration of the apparatus. In addition, the programs of FIGS. 15 to17 are stored in the memory 344 of the control unit 305 of the existingoptical route switching apparatus, and the CPU 342 executes theseprograms. Thus, the existing optical route switching apparatus can beused to achieve the operation of the optical route switching apparatus100 of the embodiment.

[0073] According to the embodiment, there are four factors to bedetected, i.e., the optical signal power failure, the error ratedeterioration, the operation clock stepping-out, and the framestepping-out. However, factors are not limited to these, and the numberof factors can be reduced/increased as occasion demands. In any case,for each of the factors, a proper masking period is preset.

[0074] Description has been made for the case where the optical routeswitching apparatus 100 of the embodiment uses, as the optical switch300, the mechanical optical switch causing an optical power failureduring the switching operation.

[0075] However, the optical route switching apparatus 100 of theembodiment can use an optical switch causing no optical power failuresduring switching. For example, an optical switch based on anelectrooptical effect and a thermooptical effect can be used. In such anoptical switch, no optical power failures occur during a switchingoperation, but operation clock stepping-out and frame stepping-out occursimilarly to the embodiment. Thus, at least an operation clockstepping-out alarm and a frame stepping-out alarm are monitored as alarmfactors, and masking for predetermined periods is applied during theswitching operation of the optical switch, thus it is made possible toprevent the operation clock stepping-out and the frame stepping-out bythe normal switching operation of the optical switch from beingerroneously recognized as occurrences of troubles. Therefore, there isno possibility of starting a new route switching operation or the likefor recovery from a trouble despite of the normal switching operation,thus making it possible to provide a highly reliable optical routesetting apparatus. Moreover, a masking period is set properly for eachalarm, then detection accuracy for an occurrence of a real trouble isnot reduced, thus it is made possible to provide a highly reliable routeswitching apparatus.

[0076] As described above, according to the present invention, it ispossible to provide a highly reliable optical communication networksystem for performing route switching of an optical signal without anychanges of the optical signal, which is capable of preventingnotification of an erroneous alarm during a switching operation of anoptical switch.

What is claimed is:
 1. An optical switching apparatus comprising: an optical switch for switching and setting routes of an optical signal without being converted; a control unit for instructing said optical switch to execute an operation of switching said routes; a performance monitor for detecting performance of the optical signal having a route set by the optical switch, and issuing an alarm; and an alarm masking unit for receiving said alarm from said performance monitor, and passing said alarm to said control unit, wherein said performance monitor issues said alarm when said performance detected is deteriorated from predetermined performance, and said alarm masking unit masks said alarm issued from said performance monitor for a predetermined masking period from a starting time of said operation of switching by said optical switch, and interrupts an input of said alarm to said control unit.
 2. The optical switching apparatus according to claim 1, wherein said performance monitor detects said performance of said optical signal for a plurality of predetermined factors, and outputs the alarm for each of said plurality of factors, said masking period of said alarm masking unit is decided for each of said plurality of factors of said performance monitor, and said alarm is masked for each of the plurality of factors.
 3. The optical switching apparatus according to claim 1, wherein said plurality of factors include at least one of optical power, operation clock synchronization, frame clock synchronization, and an error rate of said optical signal.
 4. The optical switching apparatus according to claim 2, wherein said optical switch includes output routes amounting to N in number, said performance monitor is disposed for each of the number N of output routes, said alarm is issued for said optical signal of each of said number N of output routes regarding each of said plurality of factors, and said alarm masking unit masks said alarm for each of said plurality of factors for each of said number N of output routes.
 5. The optical switching apparatus according to claim 4, wherein said alarm masking unit includes a mask register, an alarm register and an alarm issuing unit, said mask register and said alarm register have bit areas respectively corresponding to said plurality of factors for each of said number N of output routes, in said bit area of said mask register, for each of said factors of said corresponding output route, a bit indicating “MASKED STATE” is written during said masking period, and a bit indicating “MASKING RELEASED” is written when said masking is in a released state, in said bit area of said alarm register, for each of said factors of said corresponding output route, a bit indicating “THERE IS ALARM” is written when said alarm is issued from said performance monitor, and a bit indicating “NO ALARM” is written when no alarms are issued, and said alarm issuing unit issues said alarm to said control unlit when said bits respectively indicating “MASK RELEASED” and “THERE IS ALARM” are written in said corresponding bit areas of said mask register and said alarm register.
 6. An optical switch control apparatus for instructing an optical switch for switching and setting routes of an optical signal without being converted to execute a route switching operation, comprising: an alarm receiver for receiving an alarm issued from an external performance monitor for detecting performance of said optical signal having said route set by said optical switch; and an alarm masking unit for masking said alarm received by said alarm receiver for a predetermined masking period from a starting time of said switching operation of said optical switch.
 7. The optical switch control apparatus according to claim 6, wherein said external performance monitor detects said performance of said optical signal for a plurality of predetermined factors and issues said alarm for each of said plurality of factors, said masking period of said alarm masking unit is decided for each of said plurality of factors of said external performance monitor, and said alarm is masked for each of said plurality of factors.
 8. An optical switching apparatus comprising: an optical switch for switching and setting routes of an optical signal without being converted; and an alarm masking unit for masking an alarm issued from an external, performance monitor for detecting performance of said optical signal having said route set by said optical switch, wherein said alarm masking unit includes an alarm receiver for receiving said alarm issued from said external performance monitor, and masks said alarm received by said alarm receiver for a predetermined masking period from a starting time of a switching operation of said optical switch.
 9. The optical switching apparatus according to claim 8, wherein said external performance monitor detects said performance of said optical signal for a plurality of predetermined factors and issues said alarm for each of said plurality of factors, said masking period of said alarm masking unit is decided for each of said plurality of factors of said external performance monitor, and said alarm is masked for each of said plurality of factors.
 10. An optical communication network system comprising: an optical switching apparatus, wherein said optical switching apparatus includes an optical switch for switching and setting routes of an optical signal without being converted, a control unit for instructing said optical switch to execute an operation of switching said routes, a performance monitor for detecting performance of the optical signal having said route set by the optical switch, and issuing an alarm, and an alarm masking unit for receiving said alarm from said performance monitor, and passing said alarm to said control unit; said performance monitor issues said alarm when said performance detected is deteriorated from predetermined performance, and said alarm masking unit masks said alarm issued from said performance monitor for a predetermined masking period from a starting time of said operation of switching by said optical switch and interrupts an input of said alarm to said control unit.
 11. The optical communication network system according to claim 10, wherein said performance monitor detects said performance of said optical signal for a plurality of predetermined factors, and outputs said alarm for each of said plurality of factors, said masking period of said alarm masking unit is decided for each of said plurality of factors of said performance monitor, and said alarm is masked for each of said plurality of factors.
 12. An optical switching apparatus comprising: a control unit; an optical switch for setting routes of an optical signal; and a performance monitor for detecting performance deterioration of said optical signal having be set a route by said optical switch, and issuing an alarm, wherein said control unit includes means for controlling route setting of said optical switch, and means for distinguishing the alarm between (1) one caused by a trouble, and (2) one caused by the route setting operation of said optical switch and controlling issuance of said alarm to the other connected apparatus. 